Use of a coating composition containing an acid in the foundry industry

ABSTRACT

The present invention describes the use of a coating composition, comprising an aqueous phase having a pH of at most 5 and one or more refractories, in the foundry industry and also coated, waterglass-bound foundry molding elements, especially foundry molds and/or foundry cores, which each comprise such an aforementioned coating composition. The invention further describes a method for producing a coated, waterglass-bound foundry molding element. The invention likewise describes a kit whose contents include an aforementioned coating composition, comprising aqueous acid and one or more refractories.

The present invention relates to the use of a coating composition, inparticular comprising an aqueous phase having a pH of at most 5 and oneor more refractories, in the foundry, and also to coated,waterglass-bound foundry molding elements, especially foundry moldsand/or foundry cores, which each comprise a coating composition used inaccordance with the invention. The invention further relates to a methodfor producing a coated, waterglass-bound foundry molding element (moldor core). The invention likewise relates to a kit whose contents includea coating composition used in accordance with the invention. Theinvention is defined in the appended claims.

Casting in an expendable mold is a widespread method for producingnear-net-shape components, particularly in metal casting. After casting,the mold is destroyed and the casting is removed. Molds are negatives:they contain the cavity from which casting is to take place, producingthe casting whose fabrication is intended. The inner contours of thefuture casting may be formed by cores. In the production of the mold,the cavity may be molded into the molding material by means of a modelof the casting to be fabricated. Cores are usually molded in a separatecore box.

Mold base materials used for foundry molds (for the purposes of thepresent invention also called “molds”) and foundry cores (for thepurposes of the present invention also called “cores”) are predominantlygranular refractory materials such as classified silica sand that hasbeen washed. Further suitable mold base materials known per se are, forexample, zircon sands, chromite sands, chamottes, olivine sands,feldspar-containing sands, and andalusite sands. A mold base materialmay also be a mixture of different mold base materials from among thosestated, or of other preferred mold base materials. The refractory moldbase material is preferably in a free-flowing form, allowing it to beintroduced into a suitable cavity and compacted therein. The mold basematerial or the corresponding molding material mixture (moldingmaterial) is compacted in order to increase the strength of the foundrymold. To produce the foundry molds, the mold base materials are boundusing organic or inorganic molding material binders (binders). Themolding material binder generates a strong cohesion between theparticles of the molding material, providing the foundry mold with therequired mechanical stability. The production of molds and cores inindustrial practice takes place generally and advantageously in shootingmachines or molding machines, in which the particulate constituents arecompacted and the binder is cured; this is also true of the molds andcores used in the context of the present invention.

Foundry molds may be produced using either organic or inorganic moldingmaterial binders, the curing of which may respectively take place bymeans of cold or hot processes. Cold processes as referred to by theskilled person are those which are carried out substantially at roomtemperature without heating of the foundry mold. The curing in this caseis accomplished usually by a chemical reaction which is initiated, forexample, by the passage of a catalyst gas, after shaping, through themolding material mixture to be cured, this mixture comprising the moldbase material and the molding material binder. In the case of hotprocesses, the molding material mixture after shaping is heated to atemperature sufficiently high to drive out, for example the solventpresent in the molding material binder, and/or to initiate a chemicalreaction which cures the molding material binder, by crosslinking, forexample.

A feature common to all organic molding material binders, irrespectiveof their curing mechanism, is that when liquid metal is introduced intothe foundry mold, they undergo thermal decomposition and, in so doing,may release pollutants, such as, for example, benzene, toluene, xylenes,phenol, formaldehyde, and other products of thermolysis and/or cracking,some of which are unidentified. While various measures have broughtsuccess in minimizing these emissions, they nevertheless cannot beavoided completely at present in the case of organic molding materialbinders.

In order to minimize or prevent the emission of decomposition productsduring the casting procedure, molding material binders may be used whichare based on inorganic materials and contain at most a very low fractionof organic compounds. Molding material binder systems of this kind havealready been known for some considerable time, from, for example, thedocuments GB 782205 A, U.S. Pat. Nos. 6,972,059 B1, 5,582,232 A,5,474,606 A, and 7,022,178.

The term “inorganic molding material binder” refers below to a moldingmaterial binder which consists very predominantly, preferably to anextent of more than 95 wt %, more preferably more than 99 wt %, and verypreferably completely, of water and inorganic materials, so that thefraction of organic compounds in an inorganic molding material binder ofthis kind is preferably less than 5 wt %, more preferably less than 1 wt%, and very preferably 0 wt %.

The expression “inorganically bound” in the context of the present textmeans that a mold or a core has been bound with an inorganic moldingmaterial binder (as defined above).

Particularly important as a constituent of inorganic molding materialbinders is alkali metal waterglass. Alkali metal waterglass refers tovitreous, in other words amorphous, water-soluble sodium, potassium, andlithium silicates which have solidified from a melt, to mixturesthereof, and also to the corresponding aqueous solutions. The term“waterglass” below refers to those amorphous, water-soluble sodium,potassium and/or lithium silicates and/or their aqueous solutions and/ormixtures of the aforesaid silicates and/or their solutions that have ineach case a molar modulus (molar ratio) of SiO₂ to M₂O in the range from1.6 to 4.0, preferably in the range from 1.8 to 2.5, where M₂O denotesthe total amount of lithium oxide, sodium oxide, and potassium oxide.The expression “waterglass-bound” means that a foundry molding element,more particularly a mold or a core, has been produced or is producibleusing a molding material binder which comprises waterglass or consistsof waterglass. In the specification U.S. Pat. No. 7,770,629 B2, forexample, a molding material mixture is proposed which as well as arefractory mold base material comprises a waterglass-based moldingmaterial binder and a particulate metal oxide, the particulate metaloxide used being preferably precipitated silica or fumed silica.

In comparison to organic molding material binders, however, inorganicmolding material binders also have disadvantages. For example, foundrymolds or cores produced using known inorganic molding material binderspossess a comparatively low or lower stability toward atmosphericmoisture and/or toward water or aqueous moisture. It is therefore notpossible reliably, for example, for such foundry molds or cores to bestored over a prolonged period of time, as is usual with organic moldingmaterial binders.

Particularly in the context of iron and steel casting, the surfaces offoundry molding elements, more particularly of molds and cores, arecustomarily coated with a coating called a “refractory coating”,especially those surfaces which come into contact with cast metal.Refractory coatings here form a boundary layer or barrier layer betweenmold/core and metal, for the purposes, among others, of controlledsuppression of defect mechanisms at these points, or for the utilizationof metallurgical effects. Generally speaking, refractory coatings infoundry technology are intended in particular to fulfill the followingfunctions:

-   -   improving the smoothness of the surface of the casting;    -   maximizing separation of liquid metal and mold or core;    -   preventing chemical reactions between constituents of mold/core        and melt, hence facilitating the separation between mold/core        and casting; and/or    -   preventing surface defects on the casting, such as gas bubbles,        penetrations, flash and/or scabs.

The above-stated and also, where appropriate, further functions aregenerally established and optimized, and/or adapted to the particularintended purpose, by means of the precise composition of the refractorycoating or of the coating composition to be applied to the mold or thecore.

Coating compositions for use in the foundry usually comprise or arecomposed of the following components: (i) one or more fine-particledrefractories, i.e., finely particulate, refractory or highly refractoryinorganic materials, (ii) a carrier liquid comprising one or morecompounds (water, alcohols, etc.), and (iii) as further constituents,for example, one or more refractory coating binders (hereinafter alsocalled “binders” for short) and/or biocides and/or wetting agents and/orrheological additives. Ready-to-use coating compositions for the coatingof molds and cores, accordingly, are usually suspensions of finelyparticulate, refractory or highly refractory inorganic materials(refractories) in a carrier fluid, e.g., in an aqueous(water-containing) carrier liquid or a nonaqueous (water-free) carrierliquid; for details in relation to the carrier liquid, see later onbelow.

The refractory coating or the coating composition is applied to theinner contour of the casting mold or to the core by means of a suitableapplication process, such as spraying, dipping, flow coating orspreading, for example, and dried thereon to give a coating on the basisof a refractory coating, or refractory coating film. The coating on thebasis of a refractory coating may be dried by supply of heat or radiantenergy, such as by microwave radiation, for example, or by drying in theambient air. In the case of coating compositions which includecombustible compounds in the carrier liquid, the drying may also takeplace by the burning off of these compounds.

The term “refractory” in the present text and in line with the usualunderstanding of a skilled person is used to refer to compositions,materials, and minerals which are able to withstand, at least for ashort time, the temperature exposure involved in the casting or in thesolidification of an iron melt, usually cast iron. Compositions,materials, and minerals referred to as “highly refractory” are thosewhich are able briefly to withstand the casting heat of a steel melt.The temperatures which may arise during casting of steel melts areusually higher than the temperatures which may arise during the castingof iron or cast iron melts. Refractory compositions, materials, andminerals (refractories) and highly refractory compositions, materials,and minerals are known to the skilled person, from DIN 51060:2000-06,for example.

Refractories used in coating compositions are customarily mineraloxides, silicates or clay minerals. Examples of refractories alsosuitable in the context of the present invention are quartz, aluminumoxide, zirconium dioxide, aluminum silicates, phyllosilicates, zirconiumsilicates, olivine, talc, mica, graphite, coke, feldspar, diatomite,kaolins, calcined kaolins, metakaolinite, iron oxide, chromite, andbauxite, which may each be used individually or in any desiredcombinations with one another. The refractory serves, among otherthings, to seal the pores in a foundry mold or a core with respect tothe penetration of the liquid metal. Moreover, the refractory produces athermal insulation between foundry mold or core and liquid metal. Therefractory is provided usually in powder form. Unless otherwiseindicated, refractories in powder form in that case have a mean particlesize (preferably measured by light scattering according to ISO13320:2009-10) in the range from 0.1 to 500 μm, preferably in the rangefrom 1 to 200 μm. Particularly suitable as refractories are materialswhich have melting points which lie at least 200° C. above thetemperature of the particular metal melt used and/or which do not enterinto any reaction with the metal melt.

The refractories are usually dispersed in a carrier liquid. The carrierliquid is a constituent or the constituent of a coating compositionwhich is preferably in liquid form under standard conditions (20° C. and1013.25 hPa) and/or is vaporizable at 160° C. and standard pressure(1013.25 hPa). Preferred carrier liquids, which are also suitable in thecontext of the present invention, are selected from the group consistingof water and organic carrier liquids and also mixtures thereof with oneanother and/or with further constituents. Suitable organic carrierliquids are preferably alcohols, including polyalcohols and polyetheralcohols. Preferred alcohols are ethanol, n-propanol, isopropanol(2-propanol), n-butanol, and glycol. Water and aqueous mixtures(including aqueous solutions) are frequently preferred as carrierliquid.

The primary purpose of refractory coating binders (binders) is to fixthe refractories present in a coating composition on the moldingmaterial. Examples of binders which are also suitable in the context ofthe present invention are synthetic resins (organic polymers) ordispersions of synthetic resins such as polyvinyl alcohols,polyacrylates, polyvinyl acetates and/or corresponding copolymers of theaforesaid polymers. Polyvinyl alcohols are preferred. Also suitable asbinders are natural resins, dextrins, starches, and peptides.

Biocides prevent bacterial infestation. Examples of biocides alsosuitable in the context of the present invention are formaldehyde,2-methyl-4-isothiazolin-3-one (MIT),5-chloro-2-methyl-4-isothiazolin-3-one (CIT), and1,2-benzisothiazolin-3-one (BIT). The biocides, preferably the statedindividual biocides, are used customarily in a total amount of 10 to1000 ppm, preferably in an amount of 50 to 500 ppm, based in each caseon the total mass of the ready-to-use coating composition (which isintended for direct application to a casting mold or a core).

Rheological additives (standardizers) are used in order to set therefractory coating fluidity desired for processing. Inorganicstandardizers also suitable in the context of the present invention are,for example, swellable clays, such as sodium bentonite or evenattapulgite (palygorskite), for example. Examples of organicstandardizers also suitable in the context of the present inventioninclude swellable polymers, such as cellulose derivatives, moreparticularly carboxymethyl-, methyl-, ethyl-, hydroxyethyl-, andhydroxypropylcellulose; plant mucilages, polyvinylpyrrolidone, pectin,gelatin, agar agar, polypeptides and/or alginates. The aforesaidrheological additives or standardizers are preferred ingredients of thecoating composition used in accordance with the invention.

Particularly in the case of coating compositions which are aqueous(i.e., which comprise water as carrier liquid or a constituent of thecarrier liquid), moreover, it is possible to use wetting agents, inorder to achieve more effective wetting of the molding material. Theskilled person is aware of ionic and nonionic wetting agents. Ionicwetting agents used, for example, are dioctylsulfosuccinates, andnonionic wetting agents used are, for example, alkynediols orethoxylated alkynediols. The aforesaid wetting agents are also preferredingredients of the aqueous coating composition used in accordance withthe invention.

A coating composition may further comprise defoamers, pigments and/ordyes. Defoamers used may be, for example, silicone oil or mineral oil.Examples of pigments are red and yellow iron oxide and also graphite.Examples of dyes are commercial dyes known to the skilled person. Theaforesaid defoamers, pigments and/or dyes are also preferred ingredientsof the coating composition used in accordance with the invention.

In order to be able to meet the rising requirements in the area ofenvironmental and emissions protection, inorganic molding materialbinders, especially waterglass-containing molding material binders,ought in the future to gain in importance for the fabrication of moldsand cores in the sector of the casting of iron and steel as well. Toachieve the desired or necessary casting quality, it is usuallynecessary or advantageous, as indicated above, to coat inorganicallybound molds and cores with a refractory coating. In the interest ofenvironmental and emissions protection, therefore, it is logically alsoworthwhile when selecting the refractory coating to shun as far aspossible the use of organic carrier liquids and to employ preferablywater-based refractory coatings, i.e., refractory coatings with water assole carrier liquid or as at least the predominant fraction of thecarrier liquid.

As indicated above, however, foundry molding elements, especially moldsand cores, which have been produced using inorganic molding materialbinders, more particularly using waterglass-containing molding materialbinders, possess a low stability toward exposure to water or aqueousmoisture. The water present in water-based coating compositions maytherefore damage the inorganically bound molds and cores which aretreated (coated) with them. As a result, in particular, the strength ofthe molds and cores thus coated may be deleteriously diminished. Thisparticular problem which is known in foundry technology (cf., e.g., WO00/05010A1), has to date been countered only inadequately with existingmeans used, including, for example, particularly intensive curing of themolds and cores, costly and inconvenient processes for the drying of theapplied refractory coating, or the adaptation of the molding materialmixture.

The document WO 00/05010 specifies how it is possible for a coatingbased on water to be applied in particular to molds and cores that havebeen gassed with carbon dioxide and bound with sodium silicate when thecoating composition employed comprises a water-soluble or water-misciblespecific adjuvant such as esters of polyhydric alcohols, carbonates,esters or lactones. The individual constituents of the coating systemare preferably mixed with one another only immediately before thecoating procedure.

The document WO 2013/044904 A1 specifies the possibility, through thecombination of certain clays as ingredients of a water-containingrefractory coating, of producing refractory coatings having an unusuallyhigh solids content but a viscosity, nevertheless, which is comparablewith that of commercial ready-to-use refractory coatings, apparentlyenabling an improvement in the quality of the cores and molds bound withinorganic molding material binders and coated with these refractorycoatings.

The documents DE 10 2011 115 025 A1 and WO 2013/050022 A2 specify theimprovement in quality of the coated inorganic cores and molds when anaqueous coating composition is admixed with certain salts in a certainconcentration range, and particularly the apparent boost to the storagestability of said cores and molds. The salts in question are salts ofmagnesium and/or manganese, especially their sulfates and chlorides.

The documents DE 10 2011 115 024 A1 and WO 2013/050023 A2 indicate thatwhen certain additives are added to an aqueous coating composition thequality of the coated inorganic cores and molds is apparently improved,and particularly their storage stability boosted. Used as an additiveconstituent of the coating composition are esters of formic acid(methanoic acid), and the chain length of the alcohol or alcohol mixtureused in the esterification is in particular on average less than six andmore preferably less than three carbon atoms.

The document DE 27 30 753A1 describes a composition for coating moldswhich are used in the processing of molten metals, and molds coated withthis composition. The aforesaid composition may comprise, for example,formic acid or salts thereof.

The document DE 10 2006 040 385 A1 discloses temperature-stable BN moldrelease layers based on ceramic and vitreous binders; the document,however, does not disclose use for inorganically bound molds or cores(based on corresponding particulate mold base materials) for use in thefoundry.

For the priority application to the present application, the GermanPatent and Trademark Office searched the following prior art: DE 10 2006040 385 A1, DE 10 2006 002 246 A1, and DE 10 2005 041 863 A1.

As our own investigations have shown, however, the problems identifiedabove still exist to a relevant extent even in the case of a procedureaccording to the stated prior art.

Starting out from the prior art, therefore, there is a need forfurther-improved coating compositions for use in the foundry which areintended to have or to enable one or more, preferably all, of thefollowing advantageous properties:

-   -   the strength of the coated molds and/or cores producible        therewith is to be increased relative to molds and cores coated        with known water-containing refractory coatings or coating        compositions, especially where the molds and cores have been        produced with inorganic molding material binders, more        particularly with waterglass-containing molding material        binders;    -   the storage stability and also the resistance to atmospheric        moisture of the coated molds and/or cores producible therewith        is to be increased relative to molds and/or cores coated with        known water-containing refractory coatings or coating        compositions;    -   the storage stability of the coating composition itself is to        not be significantly impaired, or even to be boosted, relative        to known water-containing coating compositions;    -   the application of the coating composition to hot molds and/or        cores (i.e., in particular, to those molds and/or cores which        have temperatures of more than 50° C., preferably temperatures        in the range from 50 to 100° C.) is to be made possible or at        least improved;    -   the coated molds and cores producible therewith are to enable a        high casting quality and smoothness of the surface of the        casting, preferably a low-defect casting quality, more        preferably a defect-free casting quality;    -   the use of inorganically bound, especially waterglass-bound,        foundry molding elements, especially molds and/or cores, is to        be made possible for the casting of iron and/or steel as well,        or the possibility of use for these purposes is to be extended.

It was generally an object of the present invention to specify a coatingcomposition for use in the foundry that possesses or enables one or moreor all of the properties stated above.

It was a primary object of the present invention here to enable the useof a coating composition in the foundry that can be used oninorganically bound, in particular waterglass-bound, foundry moldingelements, preferably molds and/or cores, without adversely affectingtheir properties, in particular their strengths.

A further object of the present object was to provide coatedinorganically bound foundry molding elements, especially foundry moldsand/or foundry cores, which in each case comprise a coating compositionto be specified in accordance with the invention.

A further object of the present invention was to provide a correspondingprocess for producing an inorganically bound foundry molding elementcoated with a water-containing refractory coating.

An object of the present invention, moreover, was to provide a kit whosecontents include a coating composition specified in accordance with theinvention.

The invention is more closely defined or described in the appendedclaims. Specific and/or preferred embodiments of the invention aredescribed with more precision hereinafter. Unless otherwise indicated,preferred aspects or embodiments of the invention can be combined withother aspects or embodiments of the invention, in particular with otherpreferred aspects or embodiments. The combination of respectivelypreferred aspects or embodiments with one another again in each caseproduces preferred aspects or embodiments of the invention. Embodiments,aspects or properties which are described, or described as preferred, inconnection with the present invention for the coating composition usedin accordance with the invention are in each case also validcorrespondingly or in the same sense for processes of the invention, forcoated molds or cores of the invention, and for kits of the invention.

Where coating compositions used in accordance with the invention,processes of the invention, coated molds or cores of the invention, andkits of the invention which “comprise” or “contain” more closelyidentified embodiments, constituents or features are described below,the intention is that the corresponding variant, to be understood in anarrower scope, of the said uses, processes, coated molds or cores, orkits is in each case also intended to be disclosed, said variant“consisting” of these embodiments, constituents or features defined moreclosely in each case.

In accordance with the invention the primary object and furtherabove-indicated aspects of the general object are achieved by means ofthe use of a coating composition comprising

(a) one or more refractories, and

(b) an aqueous phase having a pH of at most 5, preferably at most 4,

for producing a coating on a waterglass-bound mold or on awaterglass-bound core, for use in the foundry, where thewaterglass-bound mold or the waterglass-bound core comprisesparticulate, amorphous silicon dioxide.

In the waterglass-bound mold or the waterglass-bound core, as well asthe particulate, amorphous silicon dioxide, there are usually relativelylarge amounts of conventional molding base materials present. Regardingthe selection of preferred mold base materials, see above.

Refractories in the coating composition (cf. constituent (a)) arepreferably one or more substances selected from the group consisting ofquartz, aluminum oxide, zirconium dioxide, aluminum silicates,phyllosilicates, zirconium silicates, olivine, talc, mica, graphite,coke, feldspar, diatomite, kaolins, calcined kaolins, metakaolinite,iron oxide, and bauxite

For the purposes of the present invention, the pH in a coatingcomposition is determined in each case from the suspension or from asuspension, preferably in accordance with the standard method DIN19260:2012-10.

The foundry molding elements which can be coated (coated) with thecoating composition of the invention may be produced in any desiredmanner known per se, as for example by shooting, pouring or by 3Dprinting technologies.

Without any assurance of correctness, it is thought that where theaqueous coating composition of the invention is used accordingly, whilethe water fraction of the coating composition does mean that bindingstructures in the alkali metal silicate framework of a waterglass-bound,coated foundry molding element (mold or core) are attacked, a furtherchemical reaction, such as an acid-base reaction, results in resultantweakenings of the binding structure that are possibly temporarily beingeliminated again, with the ultimate outcome of an increased strength onthe part of such coated, waterglass-bound foundry molding elements bycomparison with the prior art.

Preferred is a use in accordance with the invention where the coated,waterglass-bound mold or the coated, waterglass-bound core, incomparison to a coated, waterglass-bound comparative mold or a coated,waterglass-bound comparative core produced under otherwise identicalconditions using a comparative coating composition which has beenobtained from the coating composition by addition of sodium hydroxideuntil a pH of 7 has been reached, possesses a flexural strength whichdecreases less on drying. Particularly relevant technically inindustrial practice, indeed, are those “acidic” coating compositions(i.e., those comprising an aqueous phase having a pH of at most 5,preferably at most 4) whose “alkaline” equivalents (pH 7 or higher) leadto coated, waterglass-bound molds or cores which exhibit a greaterdecrease in the flexural strength on drying.

Preference is given, moreover, to a use in accordance with the inventionwhere the coated, waterglass-bound mold or the coated, waterglass-boundcore, in comparison to a coated, waterglass-bound comparative mold or acoated, waterglass-bound comparative core produced under otherwiseidentical conditions using a comparative coating composition which hasbeen obtained from the coating composition by addition of sodiumhydroxide until a pH of 7 has been reached, possess an increased storagestability.

In the context of the use of a coating composition, in accordance withthe invention, the waterglass-bound mold or the waterglass-bound corecomprises particulate, amorphous silicon dioxide.

The term “particulate, amorphous silicon dioxide” refers in the contextof the present invention to particulate synthetic silicon dioxide,preferably precipitated silica and/or fumed silicas. Fumed silicas arepreferred.

Precipitated silica is known per se and can be obtained, for example, ina manner known per se by reaction of an aqueous alkali metal silicatesolution with mineral acids: the resultant precipitate is subsequentlyseparated off, dried and, where appropriate, ground. Fumed silicas arelikewise known per se and can be obtained preferably in a manner knownper se at high temperatures by coagulation from the gas phase. Fumedsilica may be produced, for example, by flame hydrolysis of silicontetrachloride or, for the purposes of the present invention, preferablyin an arc furnace by reduction of silica sand with coke or anthracite toform silicon monoxide gas, with subsequent oxidation to form silicondioxide. A further form of amorphous, particulate silicon dioxidepreferred in accordance with the invention is obtained during theproduction of zirconium dioxide. A further possibility, known per se,for the production of particulate amorphous silicon dioxide is thespraying of a silicon dioxide melt: the primary, amorphous silicondioxide particles in this case are formed (as in other preferredproduction processes as well) not by a grinding operation.

After the production operations identified above, the primary amorphoussilicon dioxide particles (“primary particles”) are frequently inagglomerated form, i.e., are present as agglomerates of primaryparticles. The particle shape of the primary particles of theparticulate, amorphous silicon dioxide is preferably spherical. Thespherical form of the primary particles may be observed for example bymeans of scanning electron microscopy. Preferably the primary particlesof the particulate, amorphous silicon dioxide are spherical and possessa sphericity of 0.9 or more, determined by evaluation of two-dimensionalmicroscope (preferably scanning electron microscope) images.

Waterglass-bound molds and cores, including those which compriseparticulate amorphous silicon dioxide (as well as conventional mold basematerials), and their production, are known per se, from, for example,the documents WO 2006/024540 and WO 2009/056320. The aforesaid molds andcores that are known per se are suitable for the purposes of the presentinvention.

Likewise preferred, according to one embodiment, is an inventive orpreferred inventive use of a coating composition, where the aqueousphase (b) comprises

(b1) water, and

(b2) one or more acids, preferably having a pKa<5, more preferablyhaving a pKa<4,

where preferably the ratio of the mass of constituent (b1) to the massof constituent (b2) is in the range from 10:1 to 200:1, more preferablyin the range from 10:1 to 100:1.

In the production of the coating composition for use in accordance withthe invention, the one or more acids may be mixed in a customary form,i.e., in solid or liquid form and optionally diluted, preferably dilutedwith water, with further constituents of the coating composition (inother words, in particular, with refractories of constituent (a) andwater of constituent (b1)) so as to set or reach the desired pH.

The specified preferred ratio of the materials (b1) to (2) is preferablyvalid insofar as the one or more acids of constituent (b2) are selectedfrom the group of the organic acids, as described in more detail lateron below; the mass of constituent (b2) in this case relates in each caseto the total mass of the pure organic acid or organic acids (that is,without adhering material or water of crystallization, etc.). Where anacid has two or more pKa values (e.g., citric acid), the pKa referred tofor the purposes of the present invention is in each case the lowest(first) pKa.

In a further embodiment of the inventive or preferred inventive use of acoating composition, the ratio of the mass of constituent (b1) to thetotal mass of the aqueous phase (b) is preferably greater than 50%, morepreferably greater than 70%, very preferably greater than 90%.

In another preferred embodiment of the inventive or preferred inventiveuse of a coating composition, the aqueous phase preferably possesses apH of at most 4.

Preference is given generally to an inventive use wherein two or more ofthe preferred embodiments are actualized at the same time.

Particularly preferred combinations of preferred parameters, properties,and constituents of the invention are apparent generally from theappended claims.

One particularly preferred variant is an inventive or preferredinventive use of a coating composition where the constituent (b2)comprises one or more acids selected from the group consisting ofinorganic and organic acids. The aforesaid organic acids in this caseare preferably selected from the group consisting of mono-, di-, andtricarboxylic acids, preferably mono-, di-, and tricarboxylic acidswhich are solid at 25° C. and 1013 mbar (or 1013 hPa). With particularpreference the organic acids are selected from the group consisting ofcitric acid and oxalic acid. The aforesaid inorganic acids arepreferably selected from the group consisting of hydrochloric acid,nitric acid, phosphoric acid, and acidic phosphates, e.g., aluminumphosphate, more preferably from the group consisting of hydrochloricacid, nitric acid, and phosphoric acid.

The use of organic acids in combination with one or more inorganic acidsin or as constituent (b2) is preferred. Particularly preferred is theuse of organic acids in or as constituent (b2).

In the aforesaid preferred variant of the inventive use of a coatingcomposition, the ratio of the total mass of inorganic and organic acidsof constituent (b2) to the total mass of the coating composition ispreferably in the range from 0.1 to 10% (i.e., in the range from 0.1 to10 wt %), more preferably in the range from 0.5 to 5% (i.e., in therange from 0.5 to 5 wt %), more preferably still in the range from 1 to5% (i.e., in the range from 1 to 5 wt %), very preferably in the rangefrom 1 to 3.5% (i.e., in the range from 1 to 3.5 wt %), and especiallypreferably in the range from 2.5 to 3.5% (i.e., in the range from 2.5 to3.5 wt %).

Further particularly preferred is an inventive use of a coatingcomposition, preferably an inventive use designated as being preferred,where the constituent (a) comprises particulate, amorphous silicondioxide, preferably particulate, amorphous silicon dioxide whose primaryparticles (i) are spherical and/or possess a D90<10 μm, preferably a D90of <1 μm, determined by laser diffraction, more preferably particulate,amorphous silicon dioxide which as a secondary constituent comprises (i)zirconium dioxide and/or (ii) a Lewis acid, very preferably zirconiumdioxide. Preference is given to the inventive use of a coatingcomposition where the primary particles of the particulate, amorphoussilicon dioxide of the constituent (a) (i) are spherical and/or (ii)possess a D90<10 μm, preferably <1 μm, determined by laser diffraction.Preferably the primary particles of the particulate, amorphous silicondioxide of the constituent (a) (i) are spherical and possess asphericity of 0.9 or more, determined by evaluation of two-dimensionalmicroscope images. Modern commercial electron or light microscopesystems enable digital image analysis and therefore a convenientdetermination of the particle form. Digital image analysis is preferredfor studies of the sphericity.

Particularly preferred, furthermore, is an inventive use of a coatingcomposition, preferably an inventive use designated as being preferred,where the constituent (a) comprises one or more substances which areselected from the group consisting of quartz, aluminum oxide, zirconiumdioxide, aluminum silicates, phyllosilicates, zirconium silicates,olivine, talc, mica, graphite, coke, feldspar, diatomite, kaolins,calcined kaolins, metakaolinite, iron oxide, and bauxite.

The “D90” of the primary particles of the particulate, amorphous silicondioxide denotes their particle size distribution. The particle sizedistribution is determined in a manner known per se by laserdiffraction, preferably by the standard method according to DIN ISO13320:2009-10. D90 values ascertained here for the cumulative frequencydistribution of the volume-averaged size distribution function indicatethat 90 vol % of the primary particles have a particle size which is thesame as or less than the specified value (e.g. 10 μm). Suitableinstruments for determining the particle size distribution are laserdiffraction instruments known per se, of the “Mastersizer 3000” typefrom Malvern, United Kingdom, for example, preferably of the “Coulter LS230” type from Beckman Coulter, USA, the measurement being performedpreferably by means of the “Polarization Intensity DifferentialScattering” (“PIDS”) technology. With the aforesaid laser diffractiontechniques, the scattered light signals are evaluated in each casepreferably according to the Mie theory, which also takes account ofrefraction and absorption behavior of the primary particles.

Where the primary particles of the particulate, amorphous silicondioxide are present as agglomerates and/or aggregates and/or otherwiseas associations of a plurality of primary particles, these arepreferably separated gently by mechanical means or similarly in a mannerknown per se before the particle size distribution of the primaryparticles is determined, in order as far as possible to rule outdistortion of the result.

The term “secondary constituent” denotes in the context of the presentinvention that the particulate, amorphous silicon dioxide of constituent(a) includes such secondary constituents only in small amounts which mayoriginate, for instance, as impurities or adherences from upstreamproduction and/or processing procedures on the particulate, amorphoussilicon dioxide. Said secondary constituents are present preferably inan amount of not more than 18 wt % (or mass fraction), more preferablyin an amount of not more than 12 wt %, most preferably in an amount ofnot more than 8 wt %, based in each case on the total mass of theparticulate amorphous silicon dioxide of the constituent (a).

One of the aforesaid secondary constituents in the constituent (a) maybe a Lewis acid. However, it is also possible for two or more Lewisacids and/or mixtures thereof to be included. A “Lewis acid” in thecontext of the present invention is an acid according to the approachproposed by G. N. Lewis, whereby an acid is an electron pair acceptor,i.e., a molecule or ion with incomplete noble-gas configuration, whichis able to accept an electron pair provided by a Lewis base and to formwith said base a so-called Lewis adduct. A Lewis acid is electrophilic,whereas a Lewis base is nucleophilic. Consequently, molecules and ionswhich according to the conventional notions are not acids may also beinterpreted as acids.

Besides the constituents specified above, the coating compositions usedin accordance with the invention may also comprise further constituents,examples being esters, lactones, and/or acid anhydrides, examples beingmethyl formate, ethyl formate, propylene carbonate, γ-butyrolactone,diacetin, triacetin, “dibasic esters” known per se (a mixture of two ormore dimethyl esters of dicarboxylic acids, especially of glutaric acid,succinic acid, and adipic acid), acetic anhydride, methyl carbonate, andε-caprolactone.

Preference is given, in one configuration, to a use of a coatingcomposition of the invention or preferred use of a coating compositionof the invention, where the coating composition comprises one or more orall of the following constituents:

-   -   one or more biocides,    -   one or more wetting agents,    -   one or more rheological additives, and    -   one or more binders, preferably polyvinyl alcohol.

Suitable biocides include customary biocides such as microbicides,especially bactericides, algicides and/or fungicides. The biocidesindicated further above may preferably be used. Suitable wetting agentsare preferably the wetting agents recited further above. Suitablerheological additives are preferably the rheological additives recitedfurther above. Suitable binders are preferably the binders recitedabove. Polyvinyl alcohol is a particularly preferred binder.

In a further embodiment, preference is given to the inventive use of acoating composition where the coating composition possesses a solidscontent of less than 80 wt %, preferably less than 45 wt %, based ineach case on the total mass of the coating composition.

The coating composition for use in accordance with the invention ispreferably ready to use, meaning that it is intended for immediateapplication to a casting mold or to a core. Alternatively, the coatingcomposition for use in accordance with the invention may take the formof a concentrate, in which case it is then intended for dilution, inparticular by addition of water or an aqueous mixture, before beingapplied to a casting mold or to a core. This applies to all embodimentsof the present invention unless otherwise indicated or specified. Ineach individual case the skilled person decides whether a coatingcomposition is ready to use or ought additionally to be diluted.

According to a further embodiment, particular preference is given to aninventive or inventively preferred use where the coating compositioncomprises one or more binders, preferably comprising polyvinyl alcohol,in a total amount of not more than 2 wt %, preferably in an amount inthe range from 0.05 to 0.80 wt %, based in each case on the total massof the coating composition.

The solids content of a coating composition for use in accordance withthe invention is determined in the context of the present invention inaccordance with data sheet P79 from the Verein DeutscherGießereifachleute in the version of March 1976, section 6.

According to another embodiment, particular preference is given to theinventive or inventively preferred use of a coating composition wherethe coating composition is applied to a waterglass-bound mold or awaterglass-bound core for use in the casting of a metal melt with atemperature >900° C., preferably >1250° C., preferably for use in thecasting of a metal melt comprising iron and/or steel.

Particularly preferred, furthermore, is the inventive or inventivelypreferred use of a coating composition where the coating composition isapplied to a waterglass-bound mold or a waterglass-bound core for use inthe casting of iron or steel.

According to a further embodiment, particular preference is given to theinventive or inventively preferred use of a coating composition wherethe coating composition is applied to a waterglass-bound mold or awaterglass-bound core at a temperature of the waterglass-bound core orthe waterglass-bound mold of >50° C., preferably >70° C., morepreferably at a temperature <100° C. Surprisingly under these conditionsthe resulting mold or resulting core which forms or remains under theseconditions can be used for subsequent working and/or processing steps.

A further subject of the invention is the use of acid for setting a pHof at most 5, preferably a pH of at most 4, in the aqueous phase of acoating composition for application to a waterglass-bound mold or awaterglass-bound core. The aforesaid acid is preferably selected fromthe group consisting of inorganic and organic acids. The organic acidshere are preferably selected from the group consisting of mono-, di-,and tricarboxylic acids, preferably mono-, di-, and tricarboxylic acidswhich are solid at 25° C. and 1013 mbar, more preferably citric acid andoxalic acid. The inorganic acids here are preferably selected from thegroup consisting of hydrochloric acid, nitric acid, phosphoric acid, andacidic phosphates, e.g., aluminum phosphate, more preferably from thegroup consisting of hydrochloric acid, nitric acid, and phosphoric acid.In one preferred variant of the aforesaid inventive use of acid, thewaterglass-bound mold or the waterglass-bound core comprisesparticulate, amorphous silicon dioxide, and the acid is used preferablyfor setting a pH of at most 4.

Likewise a subject of the invention is a process for producing a coated,waterglass-bound mold, preferably with high storage stability, or acoated, waterglass-bound core, preferably with high storage stability,for use in the foundry, comprising the following steps:

-   (1) providing or producing a coating composition as defined    previously in the context of the use of a coating composition in    accordance with the invention,-   (2) providing or producing an uncoated, waterglass-bound mold or an    uncoated, waterglass-bound core, and-   (3) applying the provided or produced coating composition from    step (1) to the provided or produced mold or the provided or    produced core from step (2).

The coating composition produced or provided in step (1) of the processof the invention may be produced by processes that are known per se. Forexample, water in suitable quantity can be introduced initially and thefurther constituents for producing the coating composition can then beadded each in a desired amount to this initial charge with stirringusing a suitable stirrer such as a high-shear stirrer, as for example atoothed-wheel stirrer or a dissolver stirrer. If necessary, constituentsmay be incorporated in a conventional way before or during the addition.Thus, for example, optionally, one or more rheological additives may beincorporated using a high-shear stirrer, before or after addition to theinitial water charge and individually or together with one or morerefractories. If the one or more refractories are not incorporatedtogether with any added rheological additives, they may also beincorporated individually and added to the initial water charge. Afterthat, for example the further constituents of the coating compositionmay be added to the initial water charge—optionally comprisingrheological additives and/or refractories—in any order and preferablywith stirring, preferably using a high-shear stirrer, as for instanceone or more acids, optionally one or more refractory coating binders,optionally one or more biocides, optionally one or more wetting agents,optionally one or more defoamers, optionally one or more pigments and/orone or optionally more dyes.

The coating composition produced or provided in step (1) of the processof the invention may be ready for use for application to foundry moldingelements, hence being present, for example, in a concentration suitablefor use as a dipping bath for molds or cores. It is also possible forthe aforesaid coating composition, also in a conventional way, first tobe produced as a concentrate, which only later, as for example justshortly before the use of the coating composition, is diluted by means,for example, of further addition of water, to a ready-to-useconcentration (or consistency) which is then suitable for application tomolds and/or cores. If quantities or conditions are stated in thecontext of the present invention with regard to the coating compositionused in accordance with the invention, the coating composition referredto in each case is ready to use (being intended for immediateapplication to a casting mold or to a core) unless expressly statedotherwise. It is not necessary to mix the individual constituents of thecoating composition for use in accordance with the invention, with oneanother only immediately before an as-intended coating operation ontomolds or cores; instead, mixing may take place at a very much earlierstage, because the storage stability of the coating composition for usein accordance with the invention is high.

The uncoated, waterglass-bound, mold provided or produced, or theuncoated, waterglass-bound, core provided or produced, in step (2) ofthe process of the invention may be produced in a conventional way, asfor example as described in documents WO 2006/024540 or WO 2009/056320.

The applying in step (3) of the provided or produced coating compositionfrom step (1) to the provided or produced mold or the provided orproduced core may take place, according to step (2) of the process ofthe invention, in a manner known per se, preferably by the applicationmethods indicated above as being suitable, more preferably by dipping ofthe mold or the core in a coating composition used in accordance withthe invention, provided as a dipping bath.

In a preferred embodiment of the aforesaid process of the invention, theprovided or produced mold or the provided or produced core comprisesparticulate, amorphous silicon dioxide.

In a further preferred embodiment of the aforesaid process of theinvention or process preferred in accordance with the invention,application to the mold takes place at a mold or core temperatureof >50° C., preferably >70° C., more preferably at a temperature <100°C.

A further subject of the present invention is also a coated mold or acoated core for use in the foundry,

in each case comprising

-   (X) a waterglass-bound mold or a waterglass-bound core, and-   (Y) a coating comprising a coating composition as defined above in    the context of the inventive use of a coating composition.

In one preferred embodiment of this latter subject of the invention, thecoated mold or the coated core is producible by an aforesaid process ofthe invention or process preferred in accordance with the invention.

Preference is given to such an above-disclosed coated mold of theinvention or such an above-disclosed coated core of the invention wherethe waterglass-bound mold and/or the waterglass-bound core in each casecomprises particulate, amorphous silicon dioxide.

The above-disclosed mold of the invention and/or the above-disclosedcore of the invention is preferably used in the casting of a metal meltwith a temperature >900°, preferably in the casting of a metal meltcomprising iron and/or steel, more preferably in the casting of a metalmelt comprising iron and/or steel with a temperature >1250° C.

A subject of the invention is also a kit including in separatecomponents

-   (U) a coating composition comprising    -   (a) one or more refractories, and    -   (b) an aqueous phase having a pH of at most 5        for producing a coating on a waterglass-bound mold or a        waterglass-bound core, for use in the foundry,-   (V) a binder comprising waterglass, and-   (W) particulate, amorphous silicon dioxide.

In a preferred alternative, the kit of the invention comprises ascomponent (U) a coating composition, where the coating composition isdefined as above in the context of the inventive use of a coatingcomposition.

It has been found that the inventive use of the above-described coatingcomposition has in particular the following advantages over comparableor comparably used coating compositions known from the prior art, and/orsubstantiates such advantages, depending on the aspect underconsideration:

-   -   improved strength of the coated, waterglass-bound molds and/or        cores producible therewith, preferably waterglass-bound molds        and/or cores which comprise particulate, amorphous silicon        dioxide;    -   improved storage stability of the coated, waterglass-bound molds        and/or cores producible therewith, preferably waterglass-bound        molds and/or cores which comprise particulate, amorphous silicon        dioxide;    -   improved resistance to atmospheric moisture of the coated,        waterglass-bound molds and/or cores producible therewith,        preferably waterglass-bound molds and/or cores which comprise        particulate, amorphous silicon dioxide;    -   an improved possibility for application to hot molds and/or        cores (i.e. preferably to those cores and/or molds which have        temperatures of more than 50° C., preferably temperatures in the        range from 50 to 100° C.), preferably to waterglass-bound molds        and/or cores, more particularly to waterglass-bound molds and/or        cores which comprise particulate, amorphous silicon dioxide;        and/or    -   improved possibility for use of waterglass-bound foundry molding        elements, more particularly of molds and/or cores, preferably of        waterglass-bound molds and/or cores which comprise particulate,        amorphous silicon dioxide, for the casting of iron and/or steel,        by inventive use of the coating composition described.

These advantages are valid mutatis mutandis for the other subjects andaspects of the present invention.

EXAMPLES

The examples given below are intended to describe and explain theinvention in more detail without limiting its scope.

Example 1: Production of Coating Compositions

The inventive coating composition indicated in Table 1 (“SZ1”) and alsothe noninventive, comparative coating compositions (“SZ2” and “SZ3”)were produced in a conventional way by mixing the respectively indicatedingredients with one another:

For this purpose in each case the required amount of water wasintroduced initially in a glass beaker (batch size in each case around 2kg of coating composition as “concentrate”, cf. Table 1), therheological additives and the refractories (phyllosilicates, zirconflour, graphite) were added, and these ingredients were thenincorporated in a conventional way using a high-shear dissolver stirrerfor 3 minutes. The other constituents of the coating compositions (cf.Table 1) were then added in the proportions indicated, followed bystirring for a further 2 minutes with a high-shear dissolver stirrer.This gave the dilutable refractory coating-composition concentratesindicated in Table 1 in each case.

The references to “DIN grinds” in Table 1 denote that the respectivelyindicated constituent of the coating composition is present in theground state and, after the sieving of a sample of this constituent withan analytical sieve having a nominal mesh size in μm corresponding tothe stated numerical value (e.g.: “80” denotes “analytical sieve withmesh size 80 μm”) (according to DIN ISO 3310-1:2001-09), a residueremains in each case that is in the range from 1 to 10 wt %, based onthe amount of sample used.

TABLE 1 Inventive and noninventive coating compositions, each obtainedas dilutable “concentrates” Coating compositions (“concentrates”): SZ1SZ2 SZ3 Ingredients: [wt %] [wt %] [wt %] Water 44.1 46.0 47.1Rheological additive 1.5 5.0 1.5 Phyllosilicate (pyrophyllite, ./. 26.011.0 DIN 140 grind) Phyllosilicate (mica, DIN 160 25.0 ./. 18.0 grind)Zircon flour (zirconium silicate, 14.0 9.0 10.0 DIN 60 grind) Graphite(DIN 80 grind) 11.0 8.0 11.0 Polyvinyl acetate ./. 0.9 ./. Biocide(benzisothiazolinone, 0.3 0.3 0.3 10% wt/wt aqueous solution) Modifiedstarch ./. 0.3 ./. Polyvinyl alcohol 0.4 ./. 0.4 Iron oxide (yellow) ./.1.2 ./. Wetting agent 0.6 0.3 0.6 Defoamer 0.1 ./. 0.1 Propylenecarbonate ./. 3.0 ./. Citric acid 3.0 ./. ./. TOTAL: 100.0 100.0 100.0

The dilutable refractory coating-composition concentrates indicated inTable 1 above were subsequently diluted with water to produce coatingcompositions ready for use for the purpose intended here (forapplication to molds and/or cores by means of a dipping operation,preferably in the form of a dipping bath). The respective dilutionemployed and also other properties of the ready-to-use coatingcompositions resulting in each case from the dilution employed areindicated below in Table 1a:

TABLE 1a Production and properties of ready-to-use (for dipping bath ordipping tank) coating compositions Coating compositions (ready to usefor dipping tank or dipping bath): SZ1 SZ2 SZ3 Concentrate (as per Table1), 100.0 100.0 100.0 parts by weight: Water, parts by weight 40.0 30.040.0 Properties of the ready-to-use coating compositions resulting fromthe above dilution: Density [g/ml] 1.32 1.36 1.35 Flow time [s] 13.513.7 13.3 pH 2.1 7.2 6.7

As is apparent from Table 1a, the coating compositions for the purposeintended here, application to test cores by means of a dippingapplication or a dipping bath, were produced in such a way as to ensureeasy comparability (i) of their respective properties on application tothe test cores and also (ii) of the respectively resulting properties ofthe coated test cores (densities and flow times were set to be assimilar as possible; but differing pH for inventive coating compositionSZ1 relative to noninventive coating compositions SZ2 and SZ3).

The densities of the ready-to-use coating compositions, indicated inTable 1a, were measured according to the standard test method DIN EN ISO2811-2 (method A).

The flow times of the ready-to-use coating compositions, indicated inTable 1a, were measured according to the standard test method DIN 53211(1974) by determination with the DIN 4 cup.

The pH values of the ready-to-use coating compositions, indicated inTable 1a, were measured in accordance with the standard test method DIN19260:2012-10 in each case from the suspension.

The coating compositions SZ1 and SZ3 each contained attapulgite asrheological additive. Coating composition SZ2 is of the type describedin document WO00/05010.

Example 2: Investigation of the Softening of Foundry Cores

To determine the softening of foundry cores (i.e., the maximum drop inflexural strength), test cores (test specimens) were produced (inaccordance with the “core system 1” indicated in Table 4) conventionallyin a core shooting machine from Multiserw (model LUT, gassing pressure:2 bar, shot time: 3.0 s; shooting pressure 4.0 bar). An hour after thecore production, the test cores were coated (coated) with theabove-stated ready-to-use coating compositions “SZ1”, “SZ2” and “SZ3”(cf. Table 1a) at room temperature (25° C.) by dipping (conditions: 1 simmersion; 3 s hold time in the coating composition, 1 s removal). Thewet film thickness of the refractory coatings was adjusted in each caseto about 250 μm. Thereafter the coated test cores were dried in aforced-air oven under the conditions indicated below (1 hour at 120°C.), and in each case the change in their flexural strength under thedrying conditions was investigated.

The coated test cores were each dried over a period of an hour, duringwhich their flexural strengths (in N/cm², corresponding to thedefinition as indicated in data sheet R 202 of the Verein DeutscherGießereifachleute, October 1978 edition) was measured at different timesduring the drying and then once more one hour after the end of thedrying operation using a standard testing instrument of the type“Multiserw-Morek LRu-2e”, in each case with a standard measurementprogram “Rg1v_B 870.0 N/cm²” (3-point bending strength).

Table 2, for the coated test cores investigated, reports in each casethe values for the maximum dropping of flexural strength under dryingconditions, in %, based in each case on the flexural strength of therespective freshly coated (still wet) test core before the start ofdrying (initial value).

TABLE 2 Drop in strength of coated test cores under drying conditionsType of refractory Maximum drop in flexural Observation of coating onstrength on drying, core failure test core as % of the initial valueduring drying SZ1 80 No SZ2 25 No SZ 3 0 Yes

The expression “core failure” denotes here and below in each case theinvalidation of a coated core during the drying procedure, meaning thatthe coated core was in each case unusable for the measurement offlexural strength and also for a subsequently envisaged castingprocedure.

From the values reported in Table 2 it can be seen, among other things,that the maximum drop in the flexural strength of a test core coatedwith an inventive coating composition (SZ1) was significantly smallerthan with noninventive, comparative coating compositions (SZ2 and SZ3).It is further apparent from the values in Table 2 that with thenoninventive, comparative coating composition SZ3 it was not possibleunder the selected conditions to produce any usable coated cores.

Example 3: Investigation of the Storage Stability of Coated and UncoatedFoundry Cores

To determine the storage stability, waterglass-bound test cores (testspecimens) were produced in a conventional way (in the same way asdescribed in Example 2) and their flexural strengths were determined ineach case uncoated shortly after their production (one hour storagetime, relative humidity in the range from 30 to 60%, storage temperaturein the range from 20 to 25° C.) as indicated above; cf. Table 3 (entry“Uncoated after 1 h”).

Furthermore, corresponding test cores were coated as indicated below inTable 3 one hour after core production (i.e. after the same respectivetime interval from their production) at room temperature (25° C.) withthe coating compositions SZ1 and SZ3, in each case by dipping(conditions: 1 s immersion; 3 s hold time in the coating composition, 1s removal) (the coating compositions are designated as in Example 1) anddried in each case for an hour at 120° C. in a forced-air oven. Thecoated, dried test cores were then subjected to a storage test for aduration of four days (insofar as it was possible to produce the coatedcore or insofar as core failure was not observed before). Thetemperature during the storage was in each case 35° C.; the relativehumidity was in each case 75%. After the end of the storage test, theflexural strengths of the test cores were determined as indicated above.The results of these storage tests are reported below in Table 3. Thetest cores (“core system 1”) used respectively for all of the tests inExample 3 are cores whose production conditions are specified below inTable 4.

TABLE 3 Determination of the storage stability of coated and uncoatedfoundry cores Uncoated Coated with Coated with Uncoated after type SZ1after type SZ3 after on/during Core 1 h storage (4d ) storage (4 d)storage system Flexural strength [N/cm²] 1 300 149 not core failureafter determinable 131 min.

From the values reported in Table 3 it can be seen, among other things,that a waterglass-bound test core coated with an inventive coatingcomposition (SZ1) after four-day storage still had >40% of the initialstrength, whereas a test core coated with a noninventive, comparativecoating composition (SZ3) was unusable under comparable conditions; itsflexural strength was no longer determinable under the conditionsdefined above, since it fell apart during storage. Under the testconditions, an uncoated comparative core failed after just 131 min.,i.e., just the application of an inventive coating composition to a testcore resulted in the test core being stabilized under drying conditions.

TABLE 4 Production conditions for core system 1 Parameter Core system 1Molding material (100 parts by Silica sand weight) Binder (2.2 parts byweight) Alkali metal waterglass solution, 25-35 wt % waterglass contentin water (wt/wt) Additive (1.0 part by weight) Particulate, amorphoussilicon dioxide Core box temperature 120° C. Gassing temperature 150° C.Cure time  30 s

Core system 1 consisted only of the molding material, binder, andadditive constituents, as indicated in Table 4:

The binder indicated in Table 4 for core system 1 was in this case acommercial alkali metal waterglass binder “Cordis® 8511”(Hüttenes-Albertus Chemische Werke GmbH).

The additive indicated in Table 4 for core system 1 was in this case acommercial binder additive whose main constituent 95 wt %) wasparticulate, amorphous silicon dioxide, “Anorgit® 8396”(Hüttenes-Albertus Chemische Werke GmbH).

Example 4: Investigation of the Flexural Strength of Coated FoundryCores

Waterglass-bound test cores (test specimens) in each case with andwithout a content of particulate, amorphous silicon dioxide wereproduced in a conventional way (analogous to that described in Example2, but after interim maintenance of the core shooting machine used) andtheir flexural strengths were determined for comparative purposes ineach case uncoated shortly after their production (one hour storage timeat a temperature in the range from 20 to 25° C., relative humidity 30 to60%) as indicated above (for the production conditions of the testcores, see Table 6).

Furthermore, test cores as indicated below in Table 5 were coated bydipping (conditions: 1 s immersion; 3 s hold time in the coatingcomposition, 1 s removal) (designation of the coating compositions as inExample 1) and dried in each case in a forced-air oven at 120° C. for anhour. After cooling to room temperature and a storage time of 24 hours(relative humidity in the range from 30 to 60%, temperature in the rangefrom 20 to 25° C.), the flexural strengths were then determined asindicated above on the coated, dried test cores.

The results of the determinations of the flexural strengths are reportedbelow in Table 5. In this case, three different test cores (core systemsA, B and C) were used, the production conditions for each of which arereported below in Table 6.

TABLE 5 Determination of the flexural strengths of coated and uncoatedfoundry cores Uncoated Coated Coated after 1 h with type SZ1 with typeSZ3 Core system Flexural strength [N/cm²] A 401 335 production of acoated core not possible B 413* 317 production of a coated core notpossible C 347 269 production of a (without particulate, coated core notamorphous SiO2) possible * Deviation in the measured value from thecorresponding value in Table 3 for core system 1 is interpretedessentially as a consequence of the maintenance on the core shootingmachine.

From the values reported in Table 5 it can be seen that foundry corescoated with an inventive coating composition achieve high flexuralstrengths. Furthermore, the values reported in Table 5 show that with aninventive coating composition (SZ1), foundry cores produced underdifferent conditions can be successfully coated with good success (highflexural strengths). With a noninventive, comparative coatingcomposition (SZ3), on the other hand, under comparable conditions, itwas not possible to produce usable coated cores.

TABLE 6 Production conditions for core systems A, B and C Parameter Coresystem A Core system B Core system C Molding material Silica sand (100.0Silica sand (100.0 Silica sand (100.0 parts by weight) parts by weight)parts by weight) Binder Alkali metal Alkali metal Alkali metalwaterglass solution, waterglass solution, waterglass solution, 25-35 wt% 25-35 wt % 25-35 wt % waterglass content in waterglass content inwaterglass content in water (wt/wt) water (wt/wt) water (wt/wt) (2.2parts by weight) (2.2 parts by weight) (3.2 parts by weight) AdditiveParticulate, Particulate, None amorphous silicon amorphous silicondioxide dioxide (1.0 part by weight) (1.0 part by weight) Core boxtemperature 120° C. 120° C. 120° C. Gassing temperature 150° C. 150° C.150° C. Cure time  50 s  30 s  50 s

The binders and additives indicated for the core systems A, B and C inTable 6 corresponded in each case to the binders (“Cordis® 8511”) andadditives (“Anorgit® 8396”) indicated in relation to Table 4.

The above-stated core systems A, B and C consisted in each case only ofthe molding material, binder and optionally additive constituents, asindicated in Table 6.

1. The method of a coating composition comprising (a) one or morerefractories, and (b) an aqueous phase having a pH of at most 5, forproducing a coating on a waterglass-bound mold or on a waterglass-boundcore, for use in the foundry, where the waterglass-bound mold or thewaterglass-bound core comprises particulate, amorphous silicon dioxide.2. The method as claimed in claim 1, where the coated, waterglass-boundmold or the coated, waterglass-bound core, in comparison to a coated,waterglass-bound comparative mold or a coated, waterglass-boundcomparative core produced, under otherwise identical conditions, using acomparative coating composition obtained from the coating composition byadding sodium hydroxide until a pH of 7 is reached, possesses a flexuralstrength which decreases less on drying and/or possess an increasedstorage stability.
 3. The method as claimed in claim 1, where theaqueous phase (b) comprises (b1) water, and (b2) one or more acids,preferably having a pKa<5, more preferably having a pKa<4, wherepreferably the ratio of the mass of constituent (b1) to the mass ofconstituent (b2) is in the range from 10:1 to 200:1, more preferably inthe range from 10:1 to 100:1, and/or where preferably the ratio of themass of constituent (b1) to the total mass of the aqueous phase (b) isgreater than 50%, more preferably greater than 70%, very preferablygreater than 90%, and/or where preferably the aqueous phase possesses apH of at most
 4. 4. The method as claimed in claim 3, where theconstituent (b2) comprises one or more acids which are selected from thegroup consisting of inorganic and organic acids, where the organic acidsare preferably selected from the group consisting of mono-, di-, andtricarboxylic acids, preferably mono-, di-, and tricarboxylic acidswhich are solid at 25° C. and 1013 mbar, more preferably citric acid andoxalic acid, and/or where the inorganic acids are preferably selectedfrom the group consisting of hydrochloric acid, nitric acid, phosphoricacid, and acidic phosphates, more preferably from the group consistingof hydrochloric acid, nitric acid, and phosphoric acid, and/or where theratio of the total mass of inorganic and organic acids of constituent(b2) to the total mass of the coating composition is in the range from0.1 to 10%, preferably in the range from 0.5 to 5%, more preferably inthe range from 1 to 5%, very preferably in the range from in the rangefrom 1 to 3.5%, and especially preferably in the range from 2.5 to 3.5%.5. The method as claimed in claim 1, where the constituent (a) comprisesparticulate, amorphous silicon dioxide, preferably particulate,amorphous silicon dioxide whose primary particles (i) are sphericaland/or (ii) possess a D90<10 μm, preferably a D90 of <1 determined bylaser diffraction, more preferably particulate, amorphous silicondioxide which comprises as a secondary constituent (i) zirconium dioxideand/or (ii) a Lewis acid, and/or one or more substances selected fromthe group consisting of quartz, aluminum oxide, zirconium dioxide,aluminum silicates, phyllosilicates, zirconium silicates, olivine, talc,mica, graphite, coke, feldspar, diatomite, kaolins, calcined kaolins,metakaolinite, iron oxide, and bauxite.
 6. The method as claimed inclaim 1, where the coating composition comprises one or more or all ofthe following constituents: one or more biocides, one or more wettingagents, one or more rheological additives, and one or more binders,preferably polyvinyl alcohol.
 7. The method as claimed in claim 1, wherethe coating composition possesses a solids content of less than 80 wt %,preferably less than 45 wt %, based on the total mass of the coatingcomposition, and/or where the coating composition comprises one or morebinders, preferably comprising polyvinyl alcohol, in a total amount ofnot more than 2 wt %, preferably in an amount in the range from 0.05 to0.80 wt %, based on the total mass of the coating composition.
 8. Themethod as claimed in claim 1, where the coating composition is appliedto a waterglass-bound mold or a waterglass-bound core for use in thecasting of a metal melt with a temperature >900° C., preferably >1250°C., preferably for use in the casting of a metal melt comprising ironand/or steel, and/or where the coating composition is applied to awaterglass-bound mold or a waterglass-bound core for use in the castingof iron or steel and/or where the coating composition is applied to awaterglass-bound mold or a waterglass-bound core at a temperature of thewaterglass-bound core or the waterglass-bound mold of >50° C.,preferably >70° C., more preferably at a temperature <100° C.
 9. Themethod of acid for setting a pH of at most 5 in the aqueous phase of acoating composition for application to a waterglass-bound mold or awaterglass-bound core.
 10. The method as claimed in claim 9, where theacid is selected from the group consisting of inorganic and organicacids, where the organic acids are preferably selected from the groupconsisting of mono-, di-, and tricarboxylic acids, preferably mono-,di-, and tricarboxylic acids which are solid at 25° C. and 1013 mbar,more preferably citric acid and oxalic acid, and/or where the inorganicacids are preferably selected from the group consisting of hydrochloricacid, nitric acid, phosphoric acid, and acidic phosphates, morepreferably from the group consisting of hydrochloric acid, nitric acid,and phosphoric acid, and/or where the waterglass-bound mold or thewaterglass-bound core comprises particulate, amorphous silicon dioxide,where the acid is used preferably for setting a pH of at most
 4. 11. Aprocess for producing a coated, waterglass-bound mold with high storagestability, or a coated, waterglass-bound core with high storagestability, for use in the foundry, comprising the following steps: (1)providing or producing a coating composition as defined in claim 1, (2)providing or producing an uncoated, waterglass-bound mold or anuncoated, waterglass-bound core, and (3) applying the provided orproduced coating composition from step (1) to the provided or producedmold or the provided or produced core.
 12. The process as claimed inclaim 11, where the provided or produced mold or the provided orproduced core comprises particulate, amorphous silicon dioxide.
 13. Theprocess as claimed in claim 11, where application to the mold takesplace at a temperature of the core or of the mold of >50° C.,preferably >70° C., more preferably at a temperature <100° C.
 14. Acoated mold or coated core for use in the foundry, in each casecomprising (X) a waterglass-bound mold or a waterglass-bound core, and(Y) a coating comprising a coating composition as defined in claim 1.15. A coated mold or coated core, producible by the process as claimedin claim
 11. 16. The coated mold or coated core as claimed in claim 14,where the waterglass-bound mold and/or the waterglass-bound corecomprises particulate, amorphous silicon dioxide.
 17. The coated mold orcoated core as claimed in claim 14 for use in the casting of a metalmelt having a temperature >900° C., preferably for use in the casting ofa metal melt comprising iron and/or steel.
 18. A kit including inseparate components (U) a coating composition comprising (a) one or morerefractories, and (b) an aqueous phase having a pH of at most 5 forproducing a coating on a waterglass-bound mold or a waterglass-boundcore, for use in the foundry, (V) a binder comprising waterglass, and(W) particulate, amorphous silicon dioxide.
 19. A kit, comprising ascomponent (U) a coating composition, where the coating composition is asdefined in claim 1, (V) a binder comprising waterglass, and (W)particulate, amorphous silicon dioxide.